Significant countervailing forces of a resistive nature exist, for example, in the operation of a milling machine with a tool-carrying slide designed to cut a groove of predetermined length into a workpiece. Another instance, involving rotary instead of translatory motion, is a friction-welding machine in which two workpieces axially pressed together are relatively rotated to generate heat enabling their fusion under momentarily intensified axial pressure upon their arrival in a predetermined relative angular position. Such a friction-welding machine has been described, for example, in commonly owned pending application Ser. No. 339,102 filed Jan. 13, 1982 by Manfred Menzinger, now abandoned.
A complete cutoff of the energization of the drive motor upon the arrival of the load in the selected end position is generally undesirable since, aside from the shock exerted upon the load and the driven machine part, the combined inertia of that part and its load will tend to let it overshoot that position to an extent not readily ascertainable in advance. A better approach, therefore, is to reduce the driving torque to a value which lets the machine part coast to a stop more slowly and at a rate which can be empirically optimized. A friction-welding machine in which this principle is realized by purely mechanical means is disclosed in German laid-open application No. 24 36 128 according to which a shaft of a workpiece holder is initially coupled with a high-speed drive and is switched to a low-speed drive before being braked to standstill in a final position.
Such relatively cumbersome equipment is not very suitable for use in a system wherein, e.g. for friction-welding purposes, a rotary machine part is to be decelerated within a minor fraction of a second for letting the load come to rest in a stop position deviating by not more than a fraction of a degree from the desired end position.
In other systems, in which inertia rather than a countervailing resistive force is the predominant decelerating parameter, it is known to control the speed reduction of a machine part by electronic means on the basis of an operating characteristic established during an acceleration phase. With the exertion of a constant driving torque, a rotating machine part accelerates along a generally straight line or ramp characteristic to a maximum speed from which it will decelerate at substantially the same linear rate when the motor is de-energized. With the deceleration time thus known from the acceleration phase, voltage is cut off in a position suitably selected to let the load arrive with a near zero speed at the desired point where it can easily be braked to complete standstill. Such a technique, however, does not work in a situation as here contemplated in which a resistive force opposing the rotation of the driven part retards its acceleration and intensifies its deceleration so as to make the durations of the two phases significantly different from each other. A position sensor coacting with the driven part, or possibly with the load itself, would then have to take over in the terminal portion of the deceleration phase to continue the slow rotation up to the desired end position; the time required for reaching that position, starting with the instant of motor cutoff, would then be considerably lengthened. Tests have shown that, in a friction-welding machine equipped with such a control system, an acceleration phase of 300 ms is followed by a deceleration phase of 600 to 700 ms under the most favorable circumstances. This delay is inconvenient not only on account of the extended operating period but also because the frictionally generated heat of the workpieces tends to dissipate during the slow final positioning, thus before the actual welding stroke can be executed.